A pipe generally referred to as a “riser”, allowing the wellhead arranged on the sea bottom to be connected to a support at the sea surface, is used for drilling a well at sea or for producing a petroleum effluent from an offshore reservoir. A drilling or a production riser is made up of an assembly of tubular elements assembled by connectors. The tubular elements are assembled on the drilling site, from a floater. The riser is lowered into the water depth as the tubular elements are assembled, until the wellhead located on the sea bottom is reached.
In the perspective of drilling at water depths that can reach 3500 m or more, the weight of the riser becomes very penalizing. This phenomenon is increased by the fact that, for the same maximum working pressure, the length of the riser requires a larger inside diameter for the auxiliary lines, considering the necessity to limit pressure drops. Using hoop-wound tubes according to the invention as the main tube or as an auxiliary tube of a drilling riser allows to significantly reduce the weight thereof and thus to operate at greater depths. Similarly, production risers are subjected to increasingly high stresses linked with the pressure and with the weight thereof, which the present invention allows to reduce.
There are various hoop winding methods allowing a metal tube to be reinforced by arranging composite reinforcing elements, generally in form of a tape consisting of polymer-coated fibers, under stress around the metal tube. Hoop winding allows the mechanical strength of the tube to be increased without significantly increasing the weight thereof, considering the low weight of the reinforcing elements.
A hoop winding technique referred to as circumferential hoop winding consists in winding a reinforcing element around a metal tube so as to increase the internal pressure resistance of the tube. For example, document WO-82/01,159 aims to wind the reinforcing element around the metal tube while introducing a tension. Thus, the reinforcing element wound around the tube undergoes a tensional stress, which causes the metal tube to be under stress. The radial prestress undergone by the tube is similar to the prestress that would be produced by an outside pressure.
Another hoop winding method referred to as axial hoop winding aims to introduce axial compressive stresses in the metal tube and axial tensile stresses in the reinforcing element. For example, document US-2010/0,032,214 discloses a device consisting of a part that is axially mobile with respect to the metal tube so as to introduce tensile forces in the reinforcing element and compressive forces in the metal tube.
The present invention aims to associate circumferential hoop winding and axial hoop winding so as to combine the weight reductions provided by these two techniques. However, this combination poses the problem of bringing the axial hoop and the circumferential hoop under stress. Indeed, axial hoop winding mainly conditions the tensile strength of the hoop-wound tube, whereas circumferential hoop winding mainly conditions the internal pressure strength of the tube. It is important that the axial hoop winding operations do not modify, or only scarcely, the circumferential hoop winding characteristics and, conversely, that the circumferential hoop winding operations do not modify, or only scarcely, the axial hoop winding characteristics.
The present invention provides a hoop winding technique that combines a circumferential and axial reinforcement of the metal tube while allowing radial compressive prestresses to be introduced independently of the axial compressive prestresses in the metal tube.